1. Field of the Invention
The present invention relates to a projection television and, more particularly, to a projector of cathode ray tube (referred to as CRT hereinafter) of a projection type.
2. Description of the Prior Art
It is well known that a common projection television contains three, red, green, and blue color, CRTs from which an enlarged image is projected through a projection lens onto a screen. There are available two different types: a front projection type, shown in FIG. 1, and rear projection type, shown in FIG. 2, in which only one of the three CRTs is illustrated. A front projection television shown in FIG. 1 projects an image produced by the CRTs 51 through a projection lens unit 52 onto a reflective screen 53 so that a viewer can perceive a reflected light of the image. Similarly, a rear projection television shown in FIG. 2 projects an image through a projection lens unit 52 onto a transmitting screen 54. In the rear projection television, there is provided at least one reflective mirror 55 between the projection lens unit 52 and the screen 54.
In particular, the distance between the CRT 51 and the projection lens unit 52 of each projection television can be varied with ease thus compensating distortion on a projected image resulting from a change in the screen size. However, the distance between the CRT 51 and the projection lens unit 52 provides an air space 55 thus offering the following drawback.
In such an air coupling system, there are produced at least three interfaces between the air, the projection lens unit 52, and the CRT 51 due to differences in the refractive index. Each interface reflects a few percent of the beam of light emitted from the CRT 51 back to the phosphor surface of the same, whereby contrast in a projected image will be declined considerably. Also, the CRT 51 itself remains less effective in heat dissipation thus failing to increase luminance and ensuring low operational reliability and short life.
For eliminating the foregoing drawback, a modified projector assembly has been proposed as shown in FIG. 3.
The modified projector assembly shown in FIG. 3 comprises a CRT 57 for producing an image to be projected, a projection lens unit which is arranged at a distance from the CRT 57 for projecting the image and consisted mainly of e.g. four aspherical plastic lenses 58 to 61 and a glass power lens 62, a cylindrical frame 64 carrying therein the projection lens unit and coupled by an O-ring 63 to a face plate of the CRT 57 in liquid-tight arrangement, an O-ring 65 interposed between the rear end plastic lens 61 and the cylindrical frame 64 thus to provide a liquid-sealing space between the rear end lens 61 and the CRT 57 and acting as a liquid-tight means, and a cooling liquid 66 filled in the liquid-sealing space.
During operation of the above liquid cooling type projector assembly, the CRT 57 can efficiently be cooled down with the cooling liquid 66. Accordingly, luminance in the projected image will be enhanced and, also, the operational reliability and life of the CRT 57 will be increased. When the cooling liquid 66 is adapted to be equal in the refractive index to the plastic lens 61, the reflectivity of transmitting light at the interface therebetween will be declined and thus, higher contrast in the projected image will be ensured.
Such projection televisions are now available with a variety of screen sizes which correspond to different room sizes and a number of viewers to be expected. The liquid cooling type projector assemblies of corresponding sizes are hence installed in the different screen-size projection televisions. In general, the liquid cooling type projector assembly provides a narrow focusing range for image production determined strictly by the projection lens unit and will thus be applicable for limited use. Accordingly, each projection television of a given screen size has to be equipped with its corresponding size of the projector assembly, which will be disadvantageous in the reduction of production cost.